Problem: In geometry class, the girl to boy ratio is $3$ to $9$. If there are $45$ boys, how many girls are there?
A ratio of $3$ girls to $9$ boys means that there is a group of $3$ girls for every group of $9$ boys. If there are $45$ boys, then there are $5$ groups of $9$ boys each. Because we know the ratio is $3:9$ , there must be $5$ groups of $3$ girls each. There is a total of $15$ girls in geometry class.